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Humankind has recognized both Jupiter and Saturn as planets since antiquity. Jupiter, in particular, was revered as a large and important planet by ancient peoples. This recognition shows an amazing unity of perception: Imagine how long and diligently one would have to study the night sky to recognize and track the planets, especially with no telescope, and to understand that Jupiter was larger than Saturn. The ancient Egyptians (3100 to 332 b.c.e.) believed that Mars, Jupiter, and Saturn were all forms of their god Horus, a god with a man's body and a falcon's head.The Babylonians (1795 to 1595 b.c.e., and 612 to 538 B.C.E., in and around modern-day Iraq), for their part, thought of Jupiter as their god Marduk.

During antiquity, Jupiter and Saturn formed a part of mankind's understanding of the gods, toward the end of the Renaissance in the 17th century these planets reformed ideas about the solar system. In 1608 Hans Lippershey, a Dutch eyeglass maker, attached a lens to each end of a hollow tube, and thus created the first telescope. Galileo Galilei, the Italian astronomer born in Pisa in 1564, made his first telescope in 1609 from Lippershey's model.The telescope transformed the understanding of the solar system by allowing previously impossible observations of the gas giant planets. Because of their size and prominence in the solar system, Jupiter and Saturn were immediate targets for observation by the early astronomers.

Part One of this book covers what is known and still unknown about Jupiter, 400 years later in the development of telescopes and astronomy. Chapters 1,2, and 3 discuss the orbital movements of Jupiter in the solar system and its internal and external composition, movements, and temperatures. Jupiter's moons and rings (just discovered in 1979) are covered in chapter 4. Jupiter's moons have played a critical part in the development of understanding of the solar system: When Galileo discovered four of Jupiter's moons in 1610, they were the first bodies

All Orbits: Asteroid Belt, Kuiper Belt, Oort Cloud

Asteroid belt Mercury Venus Earth Mars

Asteroid belt Mercury Venus Earth Mars

The orbits of Jupiter and Saturn are highlighted here. All the orbits are far closer to circular than shown in this oblique view, which was chosen to show the inclination of Pluto's orbit to the ecliptic.

in the solar system that clearly could be demonstrated not to be orbiting Earth.These moons, Io, Europa, Ganymede, and Callisto, are still called the Galilean satellites, or Galilean moons. Their discovery marked the beginning of the end of the wildly incorrect theory that the Earth is the center of the universe and that all heavenly objects orbit this planet. Galileo began making telescopes from Venetian glass and sending them to scientists throughout Europe. He sent a telescope to Johannes Kepler, the prominent German mathematician and astronomer. Kepler did not believe that moons around Jupiter could exist until he saw them with his own eyes. When Kepler saw the moons for himself, he promptly coined the word satellite from a Latin term meaning "hangers-on to a prominent man."

Saturn, the topic of Part Two of this book, was another major target for the early astronomers. Basic data on the planet along with its orbital dynamics are covered in chapter 7. Saturn's interior structure, heat production, and magnetic field, covered in chapter 8, are all similar to Jupiter's. Saturn's weather and surface, discussed in chapter 9, are also overshadowed by dramatic Jupiter, being fainter and thus harder to see. Saturn's outstanding feature, of course, is its rings. Galileo saw Saturn's immense ring system but did not perceive correctly what he saw, writing in his notebook that Saturn appeared to have handles. Saturn's rings and moons are discussed together in chapter 10.

In 1655 Christiaan Huygens, a Dutch inventor, mathematician, and astronomer, discovered the first known moon of Saturn. Centuries of additional observation has yielded great results: At the time of writing, 33 moons have been identified.Within 10 years of Huygens's landmark discovery of the first known moon of Saturn, the banded cloud patterns and the great red spot, that huge cyclonic storm in Jupiter's southern hemisphere, were first seen. Suddenly scientists were faced with planets that had their own orbiting satellites, immense weather patterns unlike any they saw on Earth, and rings around a planet formed by processes they could not yet imagine. The distant giant planets were proving to be entirely unlike the Earth, the Moon, and Mars, and observations were pushing well past the bounds of what scientists could explain.

The excitement of continuous discovery in these planetary systems continues today. Jupiter and Saturn are the two largest planets in the solar system and the first two gas giant planets encountered when moving away from the Sun, as shown in the figure on page xviii.They are both immensely complicated systems not unlike miniature solar systems on their own: Jupiter has 63 known moons and its own ring system. Saturn is famous for its rings, and it also has 33 known moons. Astronomers are finding with some regularity new, distant moons orbiting each of these planets, so their number of known satellites will certainly continue to grow (two new moons have been identified from data sent back by the Cassini mission just during the writing of this book).

Jupiter is the largest planet in the solar system, but it contains only one-tenth of 1 percent of the mass of the Sun. If it had been only 80 times larger, it would have had the mass required to begin nuclear fusion in its interior and this solar system would have had two stars. By comparison, the Earth would have had to be 27,000 times more massive to become a star. Saturn has the youngest and freshest ring system in the solar system, presenting a beautiful laboratory for the study and understanding of rings. Each of these gas giants has a moon that has come almost to eclipse the scientific interest of the body itself: Saturn's largest moon, Titan, has an atmosphere like that of early Earth, and Jupiter's moon Europa has a water ocean and thus is the most likely place in the solar system to find life now (Mars may have had life in the past, but is less likely to have life now).

Throughout this book the two planets will be described as best they are known now, but more importantly, the directions for current research and the outstanding questions will be presented. Saturn's rings present a model for the early solar nebula and Titan's atmosphere gives an example for Earth before the development of life. Io, one of Jupiter's large moons, is the most volcanically active body on the solar system (and the only body beside Earth on which man has witnessed volcanic activity) and provides a model for the massive flood basalts that have disrupted Earth's climate in the past. A remote possibility exists that the moon Europa may harbor life in the present. Through these, and many more unusual aspects of these giant planetary systems, Earth's past, its present, and possibly its future can be studied.

Part One: Jupiter

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